1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image in accordance with an image signal or an original image, and more particularly to an image forming apparatus which is provided in a single body with a plurality of image forming means according to different methods.
2. Related Background Art
Conventionally, there have been put into practical use an image forming apparatus employing a variety of image forming means, Particularly, a page printer for forming an image by an electrophotography method is excellent in an image forming speed and silent operation and therefore widely used as a copy machine and an output device for a computer. In addition, an electrophotography apparatus capable of forming an image in a plurality of colors has also been developed for printing a multiple-color image and a pictorial image, however, it cannot be said that this type of apparatus is suitable, due to a complex mechanism, a cumbersome body, and high running and initial costs.
In the multiple-color image formation, a recording apparatus according to an ink jet method, a multi-color image forming apparatus of thermal transfer type, and so on have also been practically used in order to remove the above-mentioned defects of the electronic photography systems. Further, another image forming apparatus has been devised by combining image forming means of these different methods so as to utilize merits of the respective image forming methods. An example of such a combination type apparatus may be an image forming apparatus that forms a monochrome-image, which accounts for the greater part of image formation, by utilizing an image forming means according to the electrophotography method because of its faster image forming speed and lower running cost, and a multiple-color image by utilizing an image forming means according to the ink jet method or a thermal transfer method which is advantageous in a running cost, an initial cost, the size, and so on.
For forming a full color image by the electronic photography method, an apparatus must have a plurality of color developers for a single photo-sensitive member. It is also necessary to provide a plurality of transfer drums for sequentially carrying a recorded material to transfer color toners multiple times and parallelly arranged process kits including photo-sensitive members and developers. The apparatus, therefore, becomes extremely complicated and large in size, which results in difficulties in maintenance and an expensive running cost.
On the other hand, the image formation by the ink jet recording method and the thermal transfer method is advantageous in reducing the size of apparatus and ignorably low noise generated during a recording operation. However, improvement on productivity cannot be expected, even for monochrome or single color recording, to the image formation by the ink jet recording method in which a plurality of recording heads are reciprocally moved to perform recording.
The thermal transfer method presents difficulties in resolution and gradation, and particularly is significantly inferior to the other two recording methods in color reproductivity.
Generally, an office automation machine is required to be small in size and inexpensive. In addition, a high image quality as well as a high speed operation and good productivity are required for monochrome recording while color reproductivity and high-grade image quality are required for full color recording.
Incidentally, it is known to record a monochrome image at a high speed by the electrophotography method and thereafter paint particular colors on the image by the compact and inexpensive ink jet recording method (Japanese Laid-Open Patent Application No. 55-83079) and to record a monochrome image at a high speed by the electrophotography method and thereafter paint color portions on the image by the ink jet recording method (Japanese Laid-Open Patent Application No. 58-16857).
However, since both of the above-mentioned techniques paint certain portions on an image by the ink jet recording method after the image has been recorded by the electrophotography recording method, there are problems to be solved. Such problems will hereinafter be explained with reference to drawings used for explaining an embodiment according to the present invention.
a) If an ink jet recording unit 3 is disposed between an electronic photography recording unit 4 and a fixing means 70:
a-1) Since process speeds of the recording units 3 and 4 are largely different, a distance between the recording units 3 and 4 must be more than a maximum length of a recording sheet 52, whereby the apparatus becomes larger. To avoid this problem, the recording sheet 52 may be once accommodated in a different carrying means and then introduced into the ink jet recording unit 3. However, since an unfixed toner image is present on the recording sheet 52, this mechanism cannot be put into practice because the unfixed toner image is perturbed by carrying rollers, guides and so on.
a-2) Supposing that the ink jet recording unit 3 provides a high recording density of, for example, 400 dpi, a discharging port density of a recording head is 63.5 .mu.m. On the other hand, the diameter of a toner particle generally ranges from several .mu.m to about fifty .mu.m. Thus, if ink I is discharged onto an unfixed toner image T, as shown in FIG. 5A, the ink I impacts the toner image T, splashes out toner and consequently perturbs the toner image T.
a-3) Generally, a gap of about 1 mm or less must be provided between the recording head 31 and the recording sheet 52, and the recording sheet 52 must be maintained in an even state. It is however extremely difficult to maintain the recording sheet 52 having the unfixed toner image thereon in an even state. Contact of the recording sheet 52 on the head 31 or a paper jam may cause clogging of a discharging port, which results in a non-discharge state or a poor discharge state of the recording head 31, whereby the recording head 31 is rendered inoperable.
It is apparent that the above-mentioned troubles due to the unfixed toner image T can be completely solved by performing the ink jet recording prior to the electronic photography recording. Further, by providing the apparatus with a fan 71 (see FIG. 1) for collecting dust and exhausting heat and arranging the ink jet recording unit 3 on the upstream side of an air flow generated by the fan 71, the recording head 31 can be completely protected from adverse influences due to dispersed toner particles from a developer 44 and a cleaner 46 of the electrophotography recording unit 4. The fan 71 may be operated at least for not less than a predetermined period from the completion of the operation performed by the electrophotography recording unit 4.
Next, a comparison will be made with another arrangement.
b) If the ink jet recording unit 3 is positioned behind the fixing means 70:
b-1) Since ink jet recording is performed on the recording sheet 52 having thereon a fixed toner image T, the recording head 31 will not suffer from influences due to toner particles. However, heat from the fixer 70 or from the recording sheet 52 with the fixed image T possibly causes the ink I on the discharging face of the recording head 31 to become viscous, and accordingly the recording head 31 falls into a non-discharge disabled state or a poor discharge or a twisted state.
b-2) Generally, a main component of toner is, an agglutinative resin, for example, polyester, styreneacryl, or epoxy resin. A toner image is fixed by applying heat and pressure thereto by a pair of heat rollers. The surfaces of the heat rollers are applied with, for example, silicone oil S having an excellent separability in order to prevent toner on the recording sheet 52 from being transferred onto the heat rollers. For this reason, even in synthetic recording, such silicone oil S may adhere to the recording sheet 52, whereby ink is repelled or cannot easily reach a fiber layer inside the recording sheet 52. This problem is serious particularly for a color image which is recorded by a plurality of colors of ink. If ink does not promptly permeate into the recording sheet 52, the ink will be left wet on the sheet. When this sheet is piled on a sheet discharging tray 66, the image thereon will be perturbed by a recording sheet 52 to be next discharged (see FIG. 5B). In multiple recording for recording an image on a toner image T by ink I, particularly in an area where a toner density is high, since a recording sheet 52 is completely coated with the toner image T, the ink I can never reach the surface of the recording sheet 52, whereby the ink I runs over even during a recording operation (see FIG. 5C).